Trusses for the roof and floor of a dwelling comprise a series of lumber planks arranged in a triangulated pattern. Truss design varies from house to house because of consumer demand for individualized home design, but within a single home generally several trusses of identical configuration will be used.
The most time-consuming task of truss construction is the "set-up", which is the placement of locator stops on a truss table into positions in which they force truss planks into the proper position and orientation for subsequent attachment. Each set of planks is precut to the proper length and angle, but must be arranged on a truss table in the correct triangulated truss configuration prior to being fixed into that configuration with truss plates.
Set-up jigs are used to hasten the set-up process. They are quickly positioned on the truss table to indicate the proper placement of planks on the table. The planks are then arranged on the table, with their positions and relative orientations being defined by the placement of the set-up jigs. Semi-permanent locator stops are then placed around the planks based on the location of the planks. Generally, the truss table will include a grid of apertures which receive pins that secure the locator stops. These locator stops are considerably larger than the set-up jigs and are capable of fine translatory and angular adjustment so that pressure can be applied virtually anywhere on the planks to assure consistency between trusses.
Once the locator stops are properly positioned, the planks are attached to one another by a pressure roller which presses an attachment plate into adjoining planks to form the truss. The truss so formed is removed, and another set of planks is guided into position within the locators stops. The locator stops remain in place until all trusses of the selected configuration have been formed. They are then removed, and the set-up jigs are once again placed on the table to quickly define the configuration for the next truss configuration.
Set-up jigs in a variety of configurations are known. For example, U.S. Pat. No. 5,085,414 to Weaver discloses a jig for forming trusses which includes a block designed to fit within the rails of a C-channel sunk into and extending across the width of the table surface of a truss table. The jig can be moved along the length of the C-channel and fixed into an appropriate location through a disk-shaped stop attached to the aforementioned block. Also disclosed are a scale which fits atop and extends along the length of one of the C-channel rails and an indexing means associated with the scale to provide the operator with a reference point for positioning the jig during set-up. Another exemplary set-up jig configuration is shown in U.S. Pat. No. 4,493,038 to Harnden, which discloses a truss assembly apparatus which includes a jig positioned upon a worm gear located within a C-channel. Rotation of the worm gear causes the jig to slide within the C-channel to the desired location for set-up.
Each of these C-channel-based configurations requires that the C-channel be recessed into the table surface so that the top lips of the C-channel are level with the table surface. Although this configuration is suitable for wooden-topped truss tables, it is not suitable for the newer, more preferred steel-topped tables. The table surface of a steel-topped table has a relatively thin depth profile and is preferably provided to the operator as a single slab; each of these factors precludes the sinking of a C-channel therein. Further, the requirement that the C-channel be recessed in the table precludes moving the C-channels to different locations on the table.
In addition, the top lips of the C-channel of Weaver and the teeth of the worm gear of Harnden are prone to permanently deflect when under stress. Such stress often occurs when slightly warped planks are bent and forced into place after the set-up jigs are positioned. The forces exerted on the stops of the jigs by the deflected planks can easily be of sufficient magnitude to cause the lips of the C-channel and the worm gear teeth to deform permanently. The deformation can be sufficiently great that the jig contained therein can no longer move freely within the C-channel, and thus is no longer usable.
Accordingly, it is a first object of the present invention to provide a set-up jig suitable for use with a steel-topped truss table.
It is a second object of the present invention to provide a truss table employing such a set-up jig.
It is a third object of the present invention to provide a set-up jig and table combination that can withstand the rigors of the set-up and the truss formation operations over time despite the use of slightly warped truss planks.